Redox Chemistry
1. Redox Reactions
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🔹 Definition of Oxidation and Reduction:
🔹 Oxidation is the loss of electrons (increase in oxidation state). Reduction is the gain of electrons (decrease in oxidation state). Redox reactions involve simultaneous oxidation and reduction.
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🔹 Identification Using Oxidation States:
🔹 Assign oxidation numbers to track electron transfer. Example: In Zn + Cu²⁺ → Zn²⁺ + Cu, Zn is oxidized (0 to +2), Cu²⁺ is reduced (+2 to 0).
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🔹 Balancing Redox Equations:
🔹 Split into half-reactions (oxidation and reduction). Balance atoms and charges, equalize electrons, and combine. Example: Zn → Zn²⁺ + 2e⁻ (oxidation), Cu²⁺ + 2e⁻ → Cu (reduction).
2. Electrochemistry
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🔹 Electrolysis Overview:
🔹 Conduction through molten or aqueous electrolytes via ion movement. Requires electrodes (anode: positive, attracts anions; cathode: negative, attracts cations) and an external power source.
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🔹 Evidence for Ion Mobility:
🔹 Ions migrate to electrodes during electrolysis; e.g., color changes or gas evolution indicate ion movement.
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🔹 Electrolysis of Molten Sodium Chloride:
🔹 Molten NaCl produces sodium at the cathode (Na⁺ + e⁻ → Na) and chlorine at the anode (2Cl⁻ → Cl₂ + 2e⁻).
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🔹 Prediction of Electrolysis Products (Molten Binary Ionic Compounds):
🔹 Cations reduced at cathode to form metal; anions oxidized at anode to form non-metal. Example: PbBr₂ (molten) yields Pb at cathode, Br₂ at anode.
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🔹 Selective Discharge (Aqueous Electrolytes):
🔹 Based on reactivity series and concentration. Cathode: Less reactive cations (e.g., Cu²⁺) reduced over more reactive ones (e.g., Na⁺); high H⁺ concentration may produce H₂. Anode: Less reactive anions (e.g., SO₄²⁻) may lead to O₂ production if halides absent.
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🔹 Prediction of Aqueous Electrolyte Products:
🔹 Example: CuSO₄ (aq) with copper electrodes: Cu²⁺ + 2e⁻ → Cu at cathode; copper anode dissolves (Cu → Cu²⁺ + 2e⁻).
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🔹 Ionic Equations for Electrode Reactions:
🔹 Cathode: Reduction, e.g., Cu²⁺ + 2e⁻ → Cu. Anode: Oxidation, e.g., 2I⁻ → I₂ + 2e⁻.
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🔹 Electrolysis of Aqueous Copper(II) Sulfate for Purification:
🔹 Copper anode dissolves, impurities form sludge, pure copper deposits at cathode. Cathode: Cu²⁺ + 2e⁻ → Cu. Anode: Cu → Cu²⁺ + 2e⁻.
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🔹 Electroplating of Metals:
🔹 Example: Copper plating uses CuSO₄ electrolyte, copper anode, and object as cathode, depositing Cu via Cu²⁺ + 2e⁻ → Cu.
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🔹 Production of Electrical Energy (Simple Cells):
🔹 Galvanic cells generate electricity from redox reactions. Example: Zn/Cu cell: Zn → Zn²⁺ + 2e⁻ (anode, oxidation), Cu²⁺ + 2e⁻ → Cu (cathode, reduction).
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🔹 Hydrogen as a Fuel in Fuel Cells:
🔹 Hydrogen fuel cells combine H₂ and O₂ to produce electricity, water as byproduct: 2H₂ + O₂ → 2H₂O.
- 🔹 Zn oxidation half-reaction: Zn → Zn²⁺ + 2e⁻
- 🔹 Cu reduction half-reaction: Cu²⁺ + 2e⁻ → Cu
- 🔹 Iodide oxidation (anode): 2I⁻ → I₂ + 2e⁻
- 🔹 Permanganate reduction (acidic): MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
- 🔹 Electrolysis of molten NaCl: Cathode: Na⁺ + e⁻ → Na; Anode: 2Cl⁻ → Cl₂ + 2e⁻
- 🔹 Copper electroplating/purification: Cathode: Cu²⁺ + 2e⁻ → Cu; Anode: Cu → Cu²⁺ + 2e⁻
- 🔹 Hydrogen fuel cell: 2H₂ + O₂ → 2H₂O
- ⚠️ Confusing oxidation (electron loss) with reduction (electron gain).
- ⚠️ Assuming all ions in solution are discharged during electrolysis — selective discharge depends on reactivity and concentration.
- ⚠️ Thinking electrolysis products are the same for molten and aqueous electrolytes (e.g., NaCl aqueous produces H₂ and Cl₂, not Na).
- 👉 Clearly distinguish oxidation (loss) and reduction (gain) using OIL RIG (Oxidation Is Loss, Reduction Is Gain).
- 👉 Practice assigning oxidation states to identify redox processes.
- 👉 Balance half-reactions by ensuring equal atoms and charges before combining.
- 👉 Memorize reactivity series to predict selective discharge in electrolysis.
- 👉 Understand electrode roles: cathode (reduction), anode (oxidation).
- 👉 Be able to write ionic half-equations for common electrolysis reactions.